Thermal safety device



Oct. 22, 1940. E. HUBER THERMAL SAFETY DEVICE Filed Feb. 5, 1957 BY wEZOR. I

Patented Oct. 22, 1940 PATENT OFFICE THERMAL SAFETY DEVICE Ernst Huber,Zug, Switzerland, assignor to Landis & Gyr, A-G., a corporation ofSwitzerland Application February 5, 1937, Serial No. 124,200 InSwitzerland March 14, 1936 3 Claims.

This invention relates generally to thermal safety devicesforinterrupting the flow of current in an electrical circuit and, moreparticularly, to improvements in thermal safety devices adapted 5 foruse in low voltage electrical circuits.

Thermal safety devices for use in low voltage circuits, such as forexample, electrical circuits for house lighting and domestic electricalappliances, are well known. Such devices generally function todiscontinue the flow of current in the circuit by heating of theconductor contained within the device proper causing fusing of theconductor or energizing of a switch element. In addition, they aregenerally structurally combined with a switch element proper normallyadapted by manual or mechanical operation to open the electricalcircuit, or such safety devices are adapted to energize mechanically theswitch element proper and, by automatically opening the same, effect aninterruption of the current flow in the electrical circuit. It will beobserved that in each case the functioning of the device is dependentupon internal heating of the conductor as the result of an excess flowof current through the conductor to an external piece of electricalapparatus. Moreover, that form of construction involving mechanicalenergization of the switch element results in a complicated formation ofthe whole switching apparatus, wherein insulating intermediate parts andsprings are necessary. Both forms of construction moreover have thedisadvantage that the restoration of the device to a condition forre-use generally involves a whole series of manipulations, such asopening a casing 86 enclosing the safety device and switch element andinvolves the resoldering of a part of the circuit or the detaching of anumber of parts from the switching apparatus and the re-assemblingthereof. A further disadvantage of such devices is that their operationis directly dependent upon the current flow in the circuit in which theyform a part. In many cases, before they are actuated, damage to theelectrical equipment employed in the circuit is caused due to thegeneration of heat in such equipment before the safety devices come intooperation.

An important object of this invention is to provide a thermal safetydevice for interrupting the flow of current in a low voltage electricalcircuit which is caused to be operated by heat generated externally ofthe device as distinguished from heat generated internally of thedevice.

Another object of this invention is to provide a thermal safety devicefor use in a low voltage electrical circuit adapted to discontinue theflow of current in such circuit as the temperature of the mediumsurrounding the device reaches a predetermined safe value.

A further object of this invention is to provide a thermal safety devicefor low voltage electrical circuits adapted to interrupt the flow ofcurrent through electrical equipment in such circuits in response toheat radiating from such equipment impinging on the outer surface ofsuch device.

A still further object of this invention is to provide a safety devicefor low voltage electrical circuits such as are employed, for example,in house installations, which is of minimum dimensions, simple inconstruction, may be readily and cheaply restored to a condition forre-use and will obviate the above mentioned disadvantages.

Other and further objects of this invention will appear from thefollowing description and the appended claims.

In the accompanying drawing which forms part of the instantspecification and in which like numbers refer to like parts throughoutthe several Views:

Fig. 1 is a sectional view in elevation of a safety device forming thesubject matter of this inven- L tion, and

Fig. 2 is a sectional View in elevation of a modification of the deviceshown in Fig. 1.

In general, the safety device comprises a cartridge shaped casingenclosing a tensioned spring whose ends are connected by a flexiblecurrent conductor. The spring is held in its tensioned position by meansof electrically conducting caps of the casing, the spring being fasteneddirectly to the cap at one end and indirectly to the cap at the otherend by means of a fusible material, of which a portion is exposed to thesurrounding atmosphere. In its normally operative condition, currentflows in the circuit through the cap at one end, through the spring andconductor, through the cap at the other end and thence back into thecircuit. As the external air temperature rises, the fusible materialmelts, the spring is released thereby and in returning to its normalposition, draws with it the flexible conductor, thus interrupting thecircuit.

Referring now more particularly to Fig. l, a cylinder l formed of glassor any other suitable electrical non-conductive material is closed atthe top and bot-tom by the caps 2 and 3 respectively. The caps 2 and 3are formed of electrical conducting material such as for example copper,there being fastened to the cap 3 one end of a flexible currentconductor 4 such as a copper wire, and one end of a helical spring 5formed of steel or other suitable resilient material. The currentconductor 4 and the spring 5 are connected together at their ends andare passed through the cylinder I so that one connected pair of suchends extends into a funnel-shaped opening in the cap 2, which opening isformed by the lugs 2 on the cap 2. This opening is filled with fusiblematerial 6 which holds on to the cap 2 the corresponding end of theconductor 4, together with that of the spring 5, the latter being undertension. By fusible material it is to be understood, material that meltsat comparatively low temperature (about 176 F. to about 212 F.); it mayconsist, of an alloy containing mainly lead, such as for example, theso-called Woods metal.

The above described safety device is assumed to be connected by means ofthe caps 2-3 serving as the current conveying parts into the branch 1 ofthe line of a current consuming apparatus, so that the current isconducted through the fusible material 6 and the current conductor 4. Ifnow excessive heating of the current consuming apparatus takes place,assuming that a switch, for example, which should then open the circuitfor any reason does not come into operation, the fusible material 6melts by the extreme heat, that is, heat which is radiated ortransmitted by body contact from the currentconsuming apparatus, forexample a thermostat, and the spring 5 is releasedand draws the currentconductor 4 down in the cylinder l, whereby fiow of the current in thecircuit is discontinued.

Referring now more particularly to Fig. 2, the upper ends of the currentconductor 4 and of the spring 5 are connected to an electricallyconducting plate 8 and this plate is held by means of the fusiblematerial 6 on the cap 2. In order to apply the fusible material 6, thecap 2 is provided with an opening 9. It will be obvious that theoperation of the safety device shown in Fig. 2 is the same as that shownin Fig. 1.

Although the safety device includes a current conductor 4 in addition tothe helical spring 5, it is obvious that the conductor 4 may bedispensed with entirely if desired, and the spring 5 relied upon toeffect the transfer of current from the cap 2 to the cap 3. However,where the flow of current through the device is relatively great andinternal heatingwould result by reason of the relatively poorconductivity of the coiled steel spring as contrasted with theconductivity of the copper wire current conductor 4, it is preferredthat the conductor 4 be employed. The necessity for the non-use of theconductor 4 will be obvious to those skilled in the art in the light ofthe conditions under which the device is to be employed.

It will be obvious that the objects of this invention have beenaccomplished and there has been provided a thermal safety device ofparticular utility in low voltage electrical circuits in securing acurrent-consuming apparatus against excessive heating and damageresulting therefrom. There has been provided a thermal safety devicewhich has the advantage of great simplicity, minimum dimensions, andeasier interchangeability, which can be readily restored to operativecondition in the same manner as it was made.

It will be observed that certain features and sub-combinations are ofutility and may be employed Without reference to other features andsub-combinations. This is contemplated by and is Within the scope of theappended claims. It is further obvious that various changes may be madein details within the scope of the appended claims without departingfrom the spirit of this invention. It is, therefore, to be understoodthat this invention is not to be limited to the specific details shownand described.

Having thus described this invention, what is claimed is:

1. A thermal safety device for low voltage electrical circuitscomprising a pair of metal caps, one of said caps having an openingtherein; fusible material filling said opening; a coiled metal spring; aflexible current conductor connecting the ends of said spring; a metalplate bonded to said fusible material; said spring and conductor havingone pair of connected ends fastened to said plate and the other pair ofconnected ends attached to the other of said caps; and non-metallicinsulating means spacing said caps from each other whereby said springis normally maintained under tension.

2. A thermal safety device as claimed in claim 1, said spacing meansforming with said caps a

